Making salinity standards... Help me find a good scale

[BrettDS]

So a few months ago I got a bit frustrated trying to calibrate the conductivity probe for my apex as well as my cheap Chinese refractometer. I wound up buying three or four different brands of standards and none of them actually matched... They were all off by at least 1PPT and the highest and lowest differed by more than 3PPT.

It's time to calibrate again and I definitely don't want to go through that frustration again, so I figured the best solution might be to make my own standards using table salt by following RHF's directions here...

http://www.reefkeeping.com/issues/2004-06/rhf/index.php

In an attempt to be as accurate as possible, I'd like to do this by weight, rather than volume, so I need to get a good scale to weigh the salt and the water, but, of course, as I start thinking about that I wonder about the accuracy of the scale and how much that might affect my measurements.

Can I just run out and get a $20 or $30 kitchen scale from bed bath and beyond or would I be better off spending $100 or more on a lab scale from Amazon? Does anyone have any recommendations for a reasonably accurate (and preferably reasonably inexpensive) scale?

 

[toothybugs]

Contact a local high school and see if one of their science instructors can help you make some measurements. School science labs (at least mine) are surprisingly well-stocked on equipment. We had a balance in the basement good to 6 decimals. Max load? 1g Ask if they have an analytical balance you could use.

An offer to a fundraiser will probably help your case, but in any event, be very clear about your intent.

 

[bertoni]

Cole-Parmer has a lot of scales at varying price ranges. You could start looking there to see what your desired level of accuracy might cost. That's where I'd start.

 

[NewReefer42]

So a few months ago I got a bit frustrated trying to calibrate the conductivity probe for my apex as well as my cheap Chinese refractometer. I wound up buying three or four different brands of standards and none of them actually matched... They were all off by at least 1PPT and the highest and lowest differed by more than 3PPT.

It's time to calibrate again and I definitely don't want to go through that frustration again, so I figured the best solution might be to make my own standards using table salt by following RHF's directions here...

http://www.reefkeeping.com/issues/2004-06/rhf/index.php

In an attempt to be as accurate as possible, I'd like to do this by weight, rather than volume, so I need to get a good scale to weigh the salt and the water, but, of course, as I start thinking about that I wonder about the accuracy of the scale and how much that might affect my measurements.

Can I just run out and get a $20 or $30 kitchen scale from bed bath and beyond or would I be better off spending $100 or more on a lab scale from Amazon? Does anyone have any recommendations for a reasonably accurate (and preferably reasonably inexpensive) scale?

Hey man, I'm an analytical chemist. You can do what you're proposing, but there's no guarantee that the accuracy of the balance you use, or the salt that you use will be greater than the ability of the probe itself. In chemistry, the general rule is "your measurement will only be as good as your least accurate measurement" and often it'll be much worse than that. While you can use table salt to do what you're suggesting, it's not the best way, as table salt is not 100.00% sodium chloride, there's other crap in there. Since its not homogeneous, you will not know if you're getting a representative sample or not. One thing you could do to help this issue is make measurements in triplicate, or pentacate, and mean average them together. Also, make a concentrated solution, then serial dilute from there to reduce errors. Hth. If you want more info feel free to pm me. I'm new to reefing but not to chemistry. I'm going to check my Ca, Mg, and Fe levels via the ICPMS in my lab starting this saturday.

 

[Breadman03]

Another option may be to get a reloading scale. They measure in grains, of which there are 6999.99/pound. The RCBS 505 is about $80 and is accurate to 0.1 grains with a capacity of 511 grains or just oven an ounce. There are others out there as well.

I wouldn't sweat it too much. Randy set that up to be simple for a home hobbyist. That recipe will surely have you very close to exact, and consistency is more important than being at that exact 1.0264.

 

[Member No. 1]

Snip--->

I wouldn't sweat it too much. Randy set that up to be simple for a home hobbyist. That recipe will surely have you very close to exact, and consistency is more important than being at that exact 1.0264.

+1 - Don't sweat it.
I tried Randy's recipe as he explained using volume, then compared it to a standard, and it was spot on. I figured I just got lucky, so I tried it again, same results.

If you use Randy's recipe, so you know, you can't use it to calibrate the probe on an Apex.
I have my Apex set to display uS/cm (conductivity) which ranges 52.8-53.2 daily. I checked the tank with two separate refractometers (both calibrated with commercial standards) and both read 35ppm (1.0264) When I first tried to calibrate the probe with a 53.0 uS/cm standard, it would calibrate to 53, but when I but the probe into the tank, it would state the tank was 48-49. No matter how hard I tried or how careful I was, I could not get the Apex to read the same as my refractometers. So my work around was to calibrate the probe as the instructions state, but instead of putting the probe into the 53.0 standard, I put it into the sump, and calibrated it to the tank water. Now it reads the same as the refractometers. I only use the Apex numbers to track trends, and would never use it to control any devices. I hand check the tank weekly just to verify the Apex number. It's always the same.

 

[bertoni]

I think that the article gives a formula for producing a reasonable conductivity solution. The refractometer solution won't work at all for calibrating a conductivity meter, as noted.